Make the green (rind)-
Add 4 cups rice krispies to a large bowl, set aside.
Melt 4 cups marshmallows and 3 tablespoons butter in a large bowl in the microwave about 2 minutes, or until very puffy.
Carefully remove from microwave, stir, and add in green coloring.
Pour green marshmallow mixture over the bowl of rice krispies. Stir mixture together, quickly, using a buttered spatula.
Working quickly, divide the mixture between the two prepared cake pans, and press along the sides of the pans, creating a ring about 1.5 inches tall and 1-1.5 inches wide/thick. Rub a bit of butter on your hands as necessary, to prevent the mixture from sticking to your hands.

Make the red (watermelon) center-
Repeat as above, using the ingredients listed (left).
Press red mixture into the center of the two cake pans, press down and smoothign the top surface with a buttered spatula.

Add “seeds”
You can add chocolate covered sunflower seeds, using a tiny bit of melted chocolate to keep each seed in place.